The present invention relates to a method for the production of 2-aryl-2H-benzotriazoles by the catalytic reduction of o-nitroazobenzenes or intermediate 2-aryl-2H-benzotriazole-N-oxides. The reduction is performed with a saccharide having an aldehyde group as a reducing agent, in the presence of fluorenol as a catalyst and a base in an aqueous alcoholic solution.
2-Aryl-2H-benzotriazoles are known in the art as ultraviolet absorbing compounds useful for paints, plastics, coatings and the like, and 2-aryl-2H-benzotriazole-N-oxides are useful as intermediates for the production of 2-aryl-2H-benzotriazoles.
It is known in the art that 2-aryl-2H-benzotriazoles such as 2-phenylbenzotriazole may be produced by reducing o-nitroazobenzenes through a 2-phenylbenzotriazole-N-oxide intermediate. A wide variety of reduction techniques is known. Reduction of o-nitroazobenzenes to 2-phenylbenzotriazole by zinc in the presence of sodium hydroxide is disclosed in U.S. Pat. Nos. 3,018,269; 3,230,194; 3,773,751; 4,041,044; and 4,224,451, among others. However, this method is disadvantageous since the disposal of zinc sludge poses a significant environmental problem. The presence of residues of zinc in the benzotriazole product is detrimental to the ageing of some polymers.
Reduction with sodium hydrosulfite, sodium sulfide or sodium bisulfite are known from DE 3,731,860: EPA 130,938, and Swiss patent 660,591. Reduction by thiourea S,S-dioxide is disclosed in Japanese Kokai 61-218,577 and SU 1,159,920.
Reduction by carbon monoxide is shown in U.S. Pat. No. 4,141,903 and German Offenlegungsschrift 2,835,846. These latter processes are not preferred since they give a poor yield and are not economical.
Electrochemical reduction is disclosed in Japanese Kokai 63-186,886.
Several hydrogenation techniques are shown in German Offenlegungsschrifts 2,455,155; 2,620,970, and 2,620,897; Japanese Kokais 77-113,974 and 01-71,862; Swiss patents 615,165; 615,166 and 615,167; Canadian patents 1,154,778; 1,154,779; and European patent applications EP 380,839 and 380,840. Hydrogenation processes are carried out in an aqueous alkaline medium in the presence of finely divided platinum, nickel, palladium, rhodium or ruthenium. In hydrogenation reactions the metal catalyst serves to absorb hydrogen molecules on its surface. This absorption of hydrogen is essentially a chemical reaction where unpaired electrons on the surface of the metal mate with the electrons of hydrogen and bind the hydrogen to the surface. The collision of the nitro group from the azo-dye with the surface having absorbed hydrogen causes absorption of the nitro group as well. A step wise transfer of hydrogen atoms takes place and this produces an N-Oxide of hydroxylamine, which immediately cyclizes with the azo group and forms the N-oxide of benzotriazole. This N-oxide under the same mechanism is converted to the corresponding benzotriazole. Other reduction methods are disclosed in German Offenlegungsschrifts 2,551,853 and 2,835,529; Japanese Kokais 59-170,172; 59-172,481; 02-202,878; 02-273,677 and 02-202,877; as well as European EP 160,246.
Reduction using aldehyde reducing agents and aromatic ketone catalysts is disclosed in U.S. Pat. No. 4,835,284. Reduction using saccharides and an aromatic ketone catalyst is disclosed in U.S. Pat. Nos. 4,780,541 as well as European patent application 0,257,151. These show methods for the preparation of benzotriazoles by reductive cyclization of azo dyes with saccharides in the presence of aromatic ketone catalysts, which act by receiving hydrogen from the reducing agent and giving hydrogen to a material to be reduced. In each of these cases, saccharide reduction is catalyzed by such aromatic ketone catalysts as substituted and unsubstituted fluorenone, benzanthrone, hydroquinone, naphthoquinone, diphenoquinone, anthrone, phenanthrenequinone, anthraquinone, benzophenone, xanthenone, and the like. A disadvantage of this method is that the time of the reduction reaction and the amount of ketone catalyst is very much greater than that required for the present invention. Each of the above disclosures are incorporated herein by reference.
It has now been unexpectedly found that when reduction is done with a saccharide having an aldehyde group as a reducing agent, and fluorenol as the hydrogen transfer catalyst, which gives hydrogen to the material to be reduced and then takes hydrogen from a glucose reducing agent, that the reaction proceeds much more quickly and the amount of fluorenol required is much less compared to the use of ketone catalysts. Although the prior art shows the use of the aromatic ketone-fluorenone as a catalyst, the use of corresponding aromatic alcohol-fluorenol shows much improved results in comparison. The use of fluorenol is unique because this improvement is not apparent when other aromatic alcohols are used.